The present invention relates to a process for the draping of layers made of composite materials, as well as to apparatus for carrying out the process of the invention.
It is known that sheets made of certain composite materials comprise a multiplicity of basic filaments of some microns in diameter, combined in strands of from 1 000 to 6 000 units approximately in order to form fibres which are placed side by side (so that the materials are anistotropic), then impregnated with a resin to form sheets which vary in width and from which it is possible to cut layers or panels to drape over moulding equipment or over a layer already draped on the equipment, the number of layers to be superimposed on one another being determined by the thickness of the desired article, while the respective and relative positions, shapes and sizes of the layers are determined by the shape of the desired article, which is obtained as the outcome of a physicochemical process of solidification of the various stacked layers, for example by polymerisation under pressure in an oven.
It is also known that it is necessary for the fibres of each layer to be orientated in the direction of the stresses to which the articles being made will be subjected, in order to provide the articles with the best possible mechanical properties, and that it is therefore imperative to ensure that this orientation is carried out with great care when the articles are being produced, especially during the stacking of the successive layers on the moulds or moulding equipment.
In the present state of the art, two processes are mainly carried out for the production of articles of composite materials by draping and stacking of layers.
One of these processes is carried out entirely manually: it consists in cutting, positioning and stacking on the moulding equipment the successive layers which go to make up a particular article. All these operations are carried out by hand. It is then necessary to enclose the stack of layers thus obtained in a sealed enclosure, in which a partial vacuum is formed, in order to eliminate any risk of air being trapped between the stacked layers, and the thicker the article, and thus the greater the number of layers and possibly of reinforcements or inserts interpolated locally between two layers, the longer it is necessary to do this.
In addition, such manual draping only allows an approximate check on the orientation of the fibres of the stacked layers, particularly in the case of layers cut from a composite material with carbon fibres arranged unidirectionally, so that the mechanical properties of the article finally obtained cannot be guaranteed.
Finally, manual draping leads to long production times, and thus to excessive labour costs, which, added to high raw material costs, result in a prohibitive cost price for a not very reliable finished product, so that the ratio of output to cost achieved leads to a lack of commercial success.
The second process for producing articles made of composite materials by draping and stacking layers is basically carried out in a linear, continuous and automatic manner on one and the same machine, forming an automated and integrated computer-controlled production line.
According to this process, described in U.S. Pat. No. 4,133,711, a carrier material is arranged in a first working station, then a pre-programmed number of adjacent portions, each cut to a pre-programmed length from a web of composite material, are placed on the carrier material, their orientation being inclined with respect to a preferred direction of the carrier material, in order to produce a layer of composite material. Then the combination of the layer and the carrier material is transferred from the first working station to a second working station, at the same time as fresh carrier material is arranged in the first working station. The transferred layer is then cut according to a pre-programmed closed outline, while portions of the web of composite material, forming a new layer, are placed on the new support material in the first working station. The cutting that has been carried out is then compared with a desired cutting model and, in the event of agreement, the layer cut according to the desired model is transferred from the second working station onto a mould or moulding equipment, while the transfer of the new layer from the first working station to the second is begun.
The machine described in the above mentioned Patent, for carrying out the process described above, comprises a transfer or conveyor arm travelling in a direction perpendicular to a depositing table, and beneath said arm there is mounted a transfer head equipped with a transverse gripper capable of grasping the end of a roll of carrier material in order to unroll a length of the latter onto the depositing table, which is provided with a suction device on a level with its receiving surface for the carrier material, a cutting tool then being moved transversely over the unrolled web of carrier material in order to cut from it a piece covering the depositing table. A depositing arm is then moved perpendicularly to the depositing table and a depositing head, mounted so as to rotate beneath a platform which is itself mounted so that it is transversely movable on the arm, comprises a feed cylinder on which are wound a web of composite material and a web of backing material, a separating device which separates the backing material from the composite material during the unwinding of the feed cylinder, a device for cutting the web of composite material in order to obtain pieces of the latter which will be juxtaposed, a guide device which leads the web of composite material towards a pressing cylinder which ensures that the web of composite material comes into contact with the depositing table, and a cylinder for rewinding the backing material.
After a layer has been produced, being deposited on the piece of support material covering the depositing table, the transfer is carried out by the transfer arm, the transfer head mounted under which is equipped with a suction device, which arm is actuated perpendicular to the assembly formed by the layer and the piece of carrier material, while the suction device of the depositing table is switched off, the transfer arm then being moved perpendicular to a cutting table, also provided with an actuated suction device while the suction device of the transfer head is switched off, in order that the assembly formed by the piece of carrier material and the layer may be deposited on the cutting table, the transverse gripper with which the transfer head is provided effecting, during the movement of the transver arm, the placing of a fresh piece of carrier material to cover the depositing table.
The layer is then cut according to a pre-programmed closed outline by means of a laser arranged under a cutting arm moved perpendicular to the cutting table, while the depositing arm and the depositing head produce another layer on the new length of carrier material arranged on the depositing table.
The comparison between the cutting carried out and the desired cutting model is effected by means of a scanning device equipped with a television camera projecting the image of the cutting carried out onto a video screen on which the comparison is made. If the cutting is correct the cutting table, on which the cut layer and the length of carrier material are held by means of the suction device, moves and tilts under the control of a rotary actuator so that the cut layer is deposited on a mould or on moulding equipment arranged on a moulding table adjacent to the cutting table. A pressing or discharging device with a bar brings the layer in contact with the mould in order to prevent any floating before the suction device of the tilted cutting table is switched off.
The various moving parts of the machine are controlled by a computer synchronising their movements and directing the execution of the sequence of operations.
This machine, complex in structure, is thus expensive to buy and also requires great care in operation, hence the maintenance costs are high.
Due to its volume, the machine occupies a consequently large area of floor space and requires a large installation area for industrial production of the finished products.
In addition, it is a machine which is very suited to the manufacture of long runs of articles of large dimensions, in order to achieve the lowest possible cost price. For the production of smaller articles in limited numbers the machine would lead to a high, and even prohibitive, cost price.
Note should also be taken of the scale and cost of the preparatory programming work and, finally, that this sophisticated machine is not entirely automatic in operation, since manual intervention is provided for to remove the off-cuts from the cut layers on the cutting table before the latter is tilted through 180.degree.. This manual operation results in a slowing down of the course of operations and thus in an increase in the cost price of the finished products.